A computational study of aluminum hydroxide solvation

We have modeled aluminum hydroxide solvation using static methods HF, DFT, and MP2 and different solvaton models, as well as, Car-Parrinello molecular dynamics (CPMD). Two primary conformations were considered: Al(OH)3•H2O and Al(OH)4-. The static methods predict generally similar structures and energies, but due to the difficult modeling of hydrogen bonds to the nearest solvation shells using the continuum methods, the geometries relative to the CPMD averages are quite different. Specifically, the static methods tend to form only acceptor H bonds to the hydroxy groups. The CPMD results indicate 0.6 donors and 1.5 acceptors for each hydroxy group, the latter being slightly shorter and better defined, resulting in a total coordination number of 8-9. The ligand water forms only donor bonds, which are the strongest hydrogen bonds detected in the study. Also, in the CPMD simulations deprotonation/protonation events of these protons occurred, indicating the accessibility of both species at room temperature. The 3D environment of the hydroxy groups is tetrahedral and in general more like the solvation shell of H2O than OH-. Both vacuum and aqueous total spectra for the aluminum complexes are presented.